An X-ray computerized tomograph (hereinafter, called as an X-ray CT) is a device which reconstructs a tomogram image of a body under test from transmitted data of an X-ray obtained by irradiating an X-ray from the circumference of a body under test. An X-ray CT of a ROTATE - ROTATE system irradiates a fan beam X-ray on a body under test and detects the strength of an X-ray which transmits the body under test by use of a multi-channel X-ray detector. Accordingly, view data obtained by one irradiation become fan beam data. As one of methods of reconstructing an image based on such the view data, there is known a method in which parallel beam data are extracted from fan beam data, and an image reconstructing algorithm for a parallel beam is applied to the parallel beam data. In that case, data based on beams which are parallel with each other are picked up from fan beam data having different views with each other, thereby forming parallel beam data for each view. Therefore, data in each channel in one view have the time of irradiation of an X-ray which is different for each channel. Accordingly, in the case where an X-ray source generates, for example, a ripple-shaped output variation, data for one view include influences of such the X-ray output variation in the channel direction. The influences of such the X-ray output variation can be removed by carrying out acquisition of data in the reference channel of an X-ray detector with a sufficiently fine sampling period and carrying out correction of view data using the data in the reference channel. But, this is not effective in the case where an X-ray output decreases instantaneously due to an abnormal discharge occurring in an X-ray tube and so forth, and shower-shaped artifacts occur on the reconstructed image due to the influence of the above-mentioned decrease of the output.